1. Field of the Invention
The present invention relates to a projection tube which is used in a projection type TV receiver, a video projector or the like.
2. Description of the Related Art
An image of a cathode ray tube can be obtained by scanning an electron beam emitted from an electron gun by means of a deflection yoke. The deflection yoke is mounted in the vicinity of a joint portion between a neck and a funnel. The deflection sensitivity is enhanced as the neck outer diameter becomes smaller. However, when the neck outer diameter is made small to enhance the deflection sensitivity, the electron gun which is accommodated in the neck portion must be miniaturized correspondingly. When the electron gun is miniaturized, the diameter of an electron lens becomes small and hence, the focusing is degraded. That is, the deflection sensitivity and the focusing performance are in an opposed relationship.
A method which can solve such a problem is, for example, proposed in U.S. Pat. No. 3,163,794. In this Patent, there is disclosed a technique which enhances the deflection sensitivity by making the outer diameter of a portion of a neck of a cathode ray tube on which a deflection yoke is mounted smaller than the outer diameter of a portion of the neck in which an electron gun is accommodated. The maximum operating voltage of the cathode ray tube described in this patent is set to 16 kV.
However, such a cathode ray tube has not been commercialized yet. This is because that the maximum voltage is low so that an advantage obtained by the reduction of the deflection power is small. Further, since it is necessary to ensure a fixed dimension as the distance of the deflection yoke in the tube axis direction, when the outer diameter of a neck is set in two stages in an actual cathode ray tube, the position of an electron gun is usually made remoter from a phosphor screen due to mechanical restrictions. Accordingly, the total length of the cathode ray tube is elongated and hence, it gives rise to disadvantages such as the deterioration of the focusing performance as side effects.
On the other hand, with respect to a color cathode ray tube, in Japanese Laid-open Patent Publication 185660/1999, there is disclosed a technique which enhances the deflection sensitivity by making the outer diameter of a portion of a neck of a cathode ray tube on which a deflection yoke is mounted smaller than a portion of the neck in which an electron gun is accommodated.
However, such a cathode ray tube has not been also commercialized yet. The reason for such a circumstance is considered as follows. That is, although three electron beams which are arranged in an inline array are generated in the color cathode ray tube, since the electron beams at both sides approach an inner wall of a neck tube at a narrowed neck portion, there is a possibility that the electron beams impinge on the inner wall of the neck tube. Accordingly, it is difficult to take a large shrinkage rate of the neck diameter and hence, the deflection sensitivity enhancing effect becomes extremely small.